[wp] emit a warning when no generated goal

src/plugins/wp/Changelog

src/plugins/wp/register.ml

src/plugins/wp/tests/wp_plugin/oracle_qualif/nowp.res.oracle

...ugins/wp/tests/wp_plugin/oracle_qualif/removed.res.oracle

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src/plugins/wp/tests/wp_plugin/oracle_qualif/stmt.log

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...lugins/wp/tests/wp_region/oracle_qualif/array2.res.oracle

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...lugins/wp/tests/wp_region/oracle_qualif/array4.res.oracle

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...lugins/wp/tests/wp_region/oracle_qualif/array7.res.oracle

...lugins/wp/tests/wp_region/oracle_qualif/array8.res.oracle

...lugins/wp/tests/wp_region/oracle_qualif/fb_ADD.res.oracle

...ugins/wp/tests/wp_region/oracle_qualif/fb_SORT.res.oracle

...ugins/wp/tests/wp_region/oracle_qualif/garbled.res.oracle

...plugins/wp/tests/wp_region/oracle_qualif/index.res.oracle

...lugins/wp/tests/wp_region/oracle_qualif/matrix.res.oracle

.../wp/tests/wp_region/oracle_qualif/structarray1.res.oracle